Lathe tool

ABSTRACT

A lathe tool in which the bit is of a shape providing easy manufacture and regrinding, and a long life. The outline of the lateral faces of the bit is an elongate isosceles trapezium and an elongate isosceles triangle having as its base the longer of the two parallel sides of the trapezium, the end faces of the bit forming the equal sides of the trapezium, two inclined upper faces of the bit forming the two equal sides of the triangle and a cutting edge being formed at the junction of each upper face and the adjacent end face.

I Umted States Patent 1151 3,653,107 Hertel [451 Apr. 4, 1972 1 LATHE TOOL 3,490,117 1/1970 Hevtel ..29/96 [72] lnventor: Karl Hertel, Obedenberger Str. 29, 86 FOREIGN PATENTS OR APPLICA'UONS Nuernberg, Germany 156,320 11/1904 Germany ..29/l02 Flledi J 1 1970 896,902 11/1953 Germany ...29/105 [21] APPL N0: 2 478 1,246,360 8/1967 Germany ..29/96 Primary Examiner-Harrison L. l-linson [30] Foreign Application Priority Data Attorney-Richards & Geier Jan. Switzerland July 2, 1969 Sw1tzerland July 16, 1969 Switzerland 10860/69 A lathe I901 in which the bit is of a Shape Providing y manufacture and regrinding, and a long life. The outline of the 52 us. 0.... ..29/96, 29 95 lateral faces of the bit is an elongate isosceles trapezium and [51] Int. Cl ..B26d 1/00 an elongate isosceles triangle having as its base the longer of 58 Field of Search ..29/95 96 102 105 105 A the Parlllel Sides "aPeZium, end faces y forming the equal sides of the trapezium, two inclined upper 56] References Cited faces of the bit forming the two equal sides of the triangle and a cutting edge being formed at the junction of each upper face UNITED STATES PATENTS and the adjacent end face.

3,324,528 6/1967 Buchmann et a1. ..29/96 19 Claims, 19 Drawing Figures LATHE TOOL The invention relates to a lathe tool having a tool bit which is releasably clamped to a tool holder.

Lathe tools of this kind with multiple-edge tool bits are already known, and the tool bits can be provided with three or four cutting edges. The lateral faces of these tool bits are accordingly substantially in the form of an equilateral triangle or a square. They are secured in a recess appropriate to their external configurations on a tool holder by a screwed connection. In order to bring the next cutting edge into a cutting position after one cutting edge has become worn, the screwed connection has to be released and the bit rotated appropriately. The production of such tool bits, more particularly for parting and cutting tools, is complicated and involves a considerable proportion of rejects, since all three or four cutting edges must be identical as regards dimensions and shape and also more particularly with parting tools their cutting edges must be clean to optical standards and must be sharp. Furthermore, the overall height of the bits of tools of this kind is relatively considerable and consequently does not allow the tools equipped therewith to be of small overall height, which is necessary for many operating conditions.

Vibration free and robust fixing of the bits in known parting tools is possible only to a very limited extent. The cutting depths are also limited if the bit volume is not to be in an unfavourable ratio to the cutting bit available, and special measures are taken for securely fixing the bit. However, such additional measures for the secure fixing of bits generally result in hindering the free discharge of the chips or swarf, and this is particularly necessary for parting tools.

According to the invention there is provided a bit for a lathe tool having lateral faces which have the outline of an elongate isosceles trapezium and an elongate isosceles triangle having as its base the longer of the two parallel sides of the trapezium, the end faces of the bit forming the equal sides of the trapezium, two inclined upper faces of the bit forming the two equal sides of the triangle and a cutting edge being formed at the junction of each upper face and the adjacent end face.

The invention also provides a lathe tool comprising a tool holder, a lathe tool bit having lateral faces which have the outline of an elongate isosceles trapezium and an elongate isosceles triangle having as its base the longer of the two parallel sides of the trapezium, the end faces of the bit forming the equal sides of the trapezium, two inclined upper faces of the bit forming the two equal sides of the triangle and a cutting edge being formed at the junction of each upper face and the adjacent end face, and means for releasably clamping said tool bit on said tool holder, with one of the cutting edges projecting from the holder to form the operative cutting edge.

With such a tool, two finished cutting edges of equal width, cutting edge height and cutting length are available. The length and width of the bit can be optionally chosen without the ratio of bit volume to cutting edge length or cutting depth varying over-proportionally. The cutting tool has a support available to below the cutting edge, providing security against fracture. Single or multiple re-setting of the bit is readily possible. As a result, the bit remains fully capable of operating even after being ground several times. The tool is equally suitable as a parting tool, copying tool, threadcutting tool or knife tool. The bit shape permits simple forming in a pressure sintering process. Grinding or regrinding is simple and the overall height and width of the tool can be kept very small.

The clamping means may act on the upper face of the bit remote from the operative cutting edge, so that it does not prejudice free removal of chips or swarf.

Preferably the pressure direction of the clamping element extends perpendicularly through the bisector of the angle between the upper bit face situated remote from the operative cutting edge and the two parallel sides of said trapezium. As a result, even with a relatively large angle between the base and equal sides of the triangle, a self-locking bit clamping arrangement is achieved in such a manner that the bit is secured against being pressed out of its holder longitudinally forward.

The clamping element may be a clamping bolt which is retained within the holder and bears on the surface of the bit from the side. In one particular embodiment the clamping bolt is provided with projections abutting on the two lateral faces of the bit. In this way the bit is secured against lateral displacement. In a further development of this idea, it is proposed that the clamping bolt rests in a recess arranged at the junction of the two upper faces of the bit. As a result a further locking effect counteracts the feed pressure acting on the bit. Also free chip or swarf removal is not prevented despite the forward shifting of the clamping bolt.

The side edges of the recess may be bevelled, so that an external projection of the clamping bolt abuts on the outer bevel surface, as viewed from the tool. As a result, the clamping of the bit is substantially improved, since between that part of the clamping bolt which abuts on the bevel edge surface and the main cutting pressure receiving surface and the lateral abutment surface on the tool holder a wedge-like clamping system is provided.

To form the clearance angle of the main or secondary cutting edge, the lateral faces of the bit converge in the direction away from the upper faces. To increase the support ing surface of the bit, the central portion of the lateral faces of the bit extend approximately parallel to one another. The resulting increase in the size of the supporting surface improves the stability of the bit.

For bits of particularly narrow construction, such as are often required for parting work, the lateral faces of the bit adjacent the cutting edges are laterally offset in opposite directions relative to the vertical longitudinal central plane of the bit. This also increases the stability ofthe bit.

To improve the clamping of the bit further, the end thereof remote from the operative cutting edge may abut against a back-pressure stop. The use of the back-pressure stop is recommended more particularly when the clamping of the bit is effective on the upper face of the bit remote from the operative cutting edge. The back-pressure stop facilitates adjustment of the tool after the bit has been ground. The back-pressure stop may be the generated surface of a bolt which is mounted to be eccentrically rotatable in the holder relative to its centre axis and may be the head ofa bolt which is arranged eccentrically with respect to the centre axis. This permits a particularly inexpensive manufacture.

In a further construction, the back-pressure stop comprises a member which is freely rotatable in the tool holder about an axis substantially coincident with the cutting edge remote from the operative cutting edge and said member is provided with two abutment points abutting on the tool bit.

This feature makes it possible in a sense to convert the back-pressure acting on the bit during operation into an application pressure which acts on the face of the cutting edge situated in the inoperative position and which considerably promotes vibration-free clamping of the bit and in some cases may even make it unnecessary to provide any further clamping elements.

Preferably the back-pressure stop member is the head of a bolt screwed into the holder, and the abutment surfaces are the side boundary surfaces of a sector shaped notch formed in the bolt head. The centre of the screw head can be drilled out in order to avoid damage to the cutting edge of the bit which is in the inoperative position.

To improve stability, it is further proposed that the lower face of the bit bounded by the shorter parallel sides of the trapezia and which rests on an appropriate abutment surface of the holder, comprises two flat faces converging towards one another, and joining in a straight apex line resting on the tool holder. The converging faces form a V-shaped support and the corresponding construction of the main cutting pressure receiving surface of the tool holder makes it impossible for the clamped tool bit to be pressed off laterally. The resulting wedge-shaped association between main cutting pressure receiving surface and clamping means pressure surface further improves clamping reliability during operation.

If desired, the lateral faces are offset relatively to one another in the longitudinal direction of the bit and those sides of the triangles form the cutting edges which open into the outermost cutting edge relatively to the longitudinal direction of the bit. In this arrangement, the bit can be used as a copying tool wherein generally the main cutting edge extends in the infeed direction of the corss-slide of the copying lathe and thus parallel to the longitudinal direction of the lathe tool.

With copying tools the small overall height which is possible with the construction of the invention can be of great advantage as regards possible uses. The clamping of the bit also makes it possible to choose very small cutting edge angles for the bit without endangering clamping and positional reliability.

With particularly narrow bits, more particularly for copying purposes, it is also proposed that the entire part of a bit lateral face (partial lateral face) adjoining the back of the particular main cutting edge in the direction towards the middle of the bit extends parallel to the vertical longitudinal central plane of the bit.

In order that the invention will be better understood, the

. following description is given, merely by way of example,

reference being made to the accompanying drawings, wherein:

FIG. 1 is an exploded perspective view of one embodiment of lathe tool according to the invention;

FIG. 2 is a perspective view on a larger scale of the bit used in the tool of FIG. 1;

FIG. 3 is a side view of the tool of FIG. I assembled ready for use;

FIG. 4 is a sectional view taken on the line IVIV of FIG. 3;

FIG. Sis a view, similar to FIG. 1, ofa second embodiment;

FIG. 6 is a side view on a larger scale of the bit used in the tool of FIG.

FIG. 7 is a sectional view taken on the line VIIVII of FIG.

FIG. 8 is a view, similar to FIG. I, of a third embodiment;

FIG. 9 shows a perspective view on a larger scale of the back-pressure stop used in the embodiment of FIG. 8;

FIG. 10 is a perspective view, on a larger scale, of the bit used in the embodiment of FIG. 8;

FIG. I1 is a sectional view taken on the line XIXI of FIG. 10;

FIG. 12 is a view, similar to FIG. 10, of a modified form of bit particularly for short cutting edges;

FIG. 13 shows a side view of a further embodiment of bit, more particularly for copying purposes;

FIG. 14 shows a side view of the bit according to FIG. 13 which is rotated relatively to FIG. 13 through 180 about the side edge of its main cutting pressure supporting surface;

FIG. 15 is a perspective view of a modification of the bit of FIGS. 13 and 14;

FIG. 16 is a sectional view taken on the line XVI-XVI in FIG. 15;

FIG. 17 is a side view of the front part of a tool holder with a bit of FIG. 13 fixed therein;

FIG. 18 is a plan view on to the holder according to FIG. 17 in the direction of the arrow XVIII; and

FIG. 19 is a sectional view taken on the line XIXXIX of FIG. 17.

The lathe tool which is shown in FIG. 1, more particularly for use as a parting tool, consists of a tool holder 1, a bit shown generally at 2, an adjustable back-pressure stop 3, a clamping means or element 4 and a pressure screw 5 which operates the clamping element.

At its front end the holder 1 is provided with a recess 6, having a co-operating shape to that of the bit 2. More particularly, the holder 1 comprises a projecting rib 7 forming the main cutting pressure receiving surface 8 which extends forwards from the recess 6. The main cutting pressure receiving surface 8 supports the bit 2 over its entire base or main cutting pressure supporting surface.

The two boundary surfaces 9 of the bit 2 have the outline of an elongated isosceles trapezium 10 whose longer parallel side 1 1 forms the base of an isosceles triangle 12. The main cutting pressure supporting surface of the bit which rests on the main cutting pressure receiving surface 8 of the holder 1 and corresponds to the shorter parallel side 13 of the trapezium comprises two flat faces 14, 15 which converge downwardly to a straight apex line 16, which extends parallel to the parallel sides 11, I3 of the trapezium 10. The upper faces 16a, 17 and the end faces 18, 19 of the bit respectively are formed by equal sides of the triangle and the equal sides of the trapezium. These faces will extend at about the same angle 20 into the two boundary surfaces 9. The junction of each upper face and the adjacent end face forms a cutting edge 21, one of which projects beyond the tool holder to form the operative cutting edge.

The upper face 17 remote from the operative cutting edge is acted upon by the clamping element 4, which includes a shank 22 mounted in a bore 23 of the holder 1 and a head 24, which projects laterally above the shank 22 and bears on the upper face 17 of the bit. The longitudinal axis of the clamping element 4, which coincides with its pressure direction, extends perpendicularly to the bisector 26 of the angle between the upper face 17 and the parallel sides 11, I3 of the trapezium 10. In the region of its free end the clamping bolt 22 is provided with a notch 27, the side surface 28 of which is acted upon by the pressure screw 5 which is screwed into the holder 1. The end face 19 of the bit, remote from the operative cutting edge, abuts against the adjustable back-pressure stop 3, which is the generated surface of a bolt 31, which is mounted on the holder 1 so as to be rotatable eccentrically with respect to its center axis 33 and to be lockable in a desirable position. As illustrated the bacl -pressure stop is the head 33, arranged eccentrically with respect to the center axis 30 of a bolt which threadedly engages in a screwthreaded bore 32 in the holder 1.

FIG. 5 shows a modification in which the two upper faces 16a, 17 of the bit 2, instead of meeting in a straight apex line 34, are joined by a recess 40. Otherwise the geometric form of the bit 2' corresponds to the geometric form of the bit 2.

The main difference between holder 1 shown in FIG. 5 and that shown in FIGS. 1 to 4, is that the clamping element 4 rests with its laterally projecting part 41 in the recess 40. The part 41 of the bit 2' rests in the recess 40 and is provided with projections 42, 43 abutting laterally on the two lateral faces 9. The projections 42, 43 abut the recess 40 on two plane parallel abutment surfaces 44, 45, with a sliding fit.

In the parting tool construction shown in FIG. 5, an adjustable back'pressure stop is not absolutely necessary for the reliable holding of the bit 2'. Here the back-pressure stop 46 is a bolt which is secured on the holder 1 extending approximately in the direction of the longitudinal central plane 75 of the bit and the main cutting pressure component, the said bolt having its generated surface facing towards the bit provided with a V- shaped or sector-shaped notch 46a in such a manner that the end face 19 and the upper face 17 of the bit remote from the operative cutting edge abut on the two side surfaces 46b and 460 of the notch 46a. The bolt 46 is mounted in the bore 464 of the holder 1 and is anchored therein by a transverse bolt 46e, which extends diametrally through the bolt 46 and is itself screwed to the holder 1. The transverse bolt 46a extends through the bolt 46 with considerable play permitting slight rotational and translational movement of the bolt 46 within the bore 46d. This enables an automatic adaptation of the back-pressure bolts 46 to particular manufacturing inaccuracies of the bit 2'.

The lathe tool which is shown in FIG. 8 and which is also especially suitable for parting purposes is constructed similarly to the lathe tool shown in FIGS. 1 and 5. It comprises a tool holder 58, a bit 59, a back-pressure stop 50, a clamping bolt 60 and a pressure screw 61 operating the clamping bolt 60.

The holder 58 is provided at its front end with a recess 62, having an outline corresponding to that of the bit 59. Thus, the holder 58 is provided with a projecting rib 63 providing the main cutting pressure receiving surface 64, which supports the bit 59 over its entire lower surface 66.

The bit 59 is similar to the bit shown in FIG. 5, but the lower face 66 is plane, and the side surfaces of the recess 67 are bevelled in such a manner that larger bevel edge surfaces 68 are formed. The bevel edge surfaces 68 extend at an obtuse angle into the part-cylindrical surface 69 of the recess 67. The inner surface 70 of the clamping bolt projection 71 has an inclination corresponding to the bevel edge surfaces 68 in such a manner that the projection 71 is in surface contact with the bevel edge 68.

In the central portion situated below the recess 67 the lateral faces 65 of the bit are situated parallel to one another, as with the bits shown in FIG. 1, FIG. 2, FIG. 6 and FIG. 7 (surfaces 72).

In the embodiment shown in FIG. 12 the faces 73, 74 of the bit 59 are offset laterally in opposite directions to one another relatively to the vertical longitudinal central plane 75 of the bit. Otherwise the bit construction corresponds to the construction shown in FIG. 10.

The parts 60, 61 (FIGS. 8 and 11) necessary for the clamping of the bit 59 operate in corresponding manner to the corresponding parts 4, 5 described in more detail with reference to FIG. 5.

In the tool holder shown in FIG. 8 a particularly advantageous back-pressure stop 50 has been used, which can be used equally with the tools or bits shown in FIGS. 1 to 7. It is mounted on the holder 58 to be freely rotatable about an axis 51 extending approximately at right angles to the longitudinal central plane of the bit 59. The axis 51 coincides approximately with the bit cutting edge which is in the inoperative position. The back-pressure stop 50 comprises two supporting points which adjoin the end face 19 and upper face 17 of the bit, the points being formed on the side boundary surfaces 52, 53 of a V-shaped or sector-shaped notch 55 in the head 54 ofa screw threaded into a bore 32 of the holder 58. The screw head is axially bored at 56 in order to surround the bit cutting edge which is in the inoperative position with contact and without any risk of damage. The head 54 is provided with a slot 57, to facilitate screwing into the holder 1.

The two lateral faces 75' of the bit 76 shown in FIGS. 13 and 14 have the outline form of an elongated trapezium 77 whose longer parallel side 78 forms the base of an isosceles triangle 79. The two lateral faces 75' of the bit 76 are offset relatively to one another to the extent 83 in the longitudinal direction 82 of the bit. The offsetting direction in this embodiment is so chosen that a left-hand cutting bit is provided. If the offsetting is in the opposite direction, a right-hand cutting bit or tool is provided. Depending on the amount 83 ofthe mutual displacement of the boundary surfaces relatively to one another, the angle 84 between the faces 85, 86 of the bit 76 and the vertical bit longitudinal central plane 87 varies. Since the edges 88, 89 are the two cutting edges of the bit 76, therefore, the amount 83 of the relative longitudinal displacement of the side boundary surface 75 in the longitudinal direction 16 determines the amount of the angle of rake of the bit. The cutting edge point associated with the bit cutting edge 88, 89 are designated as 90, 91. The front end surfaces 92, 93 of the bit 76 are the backs of the secondary cutting edges 94, 95. The amount 83 of the relative longitudinal displacement of the lateral faces 75 also influences the amount of the angle of rake 96 or 97 of the bit.

The clamping bolt 98 coincides as regards its operation with the clamping bolt shown in FIGS. 1-11. It also rest on the particular face 85 of the bit 76 which happens to be situated in the inoperative position. Owing to the angle of rake (similar to the angle 84) of the particular face 85 which is in the inoperative position, with surface contact of the head of the clamping bolt 98 on the bit side which is in the inoperative position, a pressure is exerted which forces the bit 76, in the manner ofa wedge, into the angle between the main cutting pressure receiving surface 99 and the feed pressure supporting surface 100.

101 is the transition between one part 102 of each lateral face 75 and the other part 103. The partial lateral faces designated as 102, are the back of the cutting edges 88, 89. Since the back of each cutting edge deviates from the vertical in order to form the clearance angle, the position of the partial lateral faces 102 of the bit 76 also deviates from the vertical in the manner shown in FIG. 19. In contrast, the partial side boundary surfaces 103 extend parallel to the vertical longitudinal central plane 87 of the bit. Whereas in the bit according to FIGS. 13, 14 and I7 the main cutting pressure supporting surface is plane, in the bit according to FIGS. 15 and 16 a pair of downwardly convergent flat faces is provided for the main cutting pressure supporting surface 105.

The feed pressure supporting surface extends to the forward boundary edge of the back of the secondary cutting edge of the bit, as the plan view in FIG. 18 shows, Otherwise the form of the recess in the holder or the surface of the back-pressure stop is adapted to the contour of the adjoining part of the bit.

A particularly simple embodiment of the bit 76 is obtained if the two lateral boundary surfaces 75' extend in plane parallel manner relatively to one another over their entire length. This is possible since the bit 76 is to be used primarily for a cutting tool and not for a parting tool. For producing the necessary clearance angle, the support of the bit 76 need only be slightly inclined in such a manner that the bit 76 is swung to the extent of the desired clearance angle in the clockwise direction (FIG. 19) about its longitudinal axis which is disposed at right angles to the plane of the drawing.

The back-pressure stop 3' is constructed as a stationary element in FIGS. 17 and 18. However, preferably it is also proposed with the bit 76 to use as the back-pressure stop 8 a bolt which is constructed in accordance with the bolt 46 (FIG. 5), and whose V-shaped notch 46a corresponds geometrically to the cutting edge 94 or of the bit 76 which is in the inoperative position.

Iclaim:

l. A lathe tool comprising, in combination:-

a. a tool holder;

b. a tool bit on said tool holder;

c. lateral faces to said tool bit, having the outline of an elongate isosceles trapezium comprising two equal sides and longer and shorter parallel sides and an elongate isosceles triangle having as its base the longer of said parallel sides; d. end faces to said bit forming the equal sides of said isosceles trapezium;

e. two upper faces to said bit forming the equal sides of said isosceles triangle;

. a cutting edge to said bit at the junction of each upper face and the adjacent face; and

g. means releasably clamping said tool bit on said tool holder, with one of said cutting edges projecting from said holder to form the operative cutting edge.

2. A lathe tool as claimed in claim I, wherein at least a portion of said lateral faces converge towards one another away from said upper faces to form side clearance angles.

3. A lathe tool as claimed in claim 2, wherein the central portions of the laterial faces are substantially parallel to one another.

4. A lathe tool as claimed in claim 1, wherein the laterial faces are offset relatively to one another in the longitudinal direction of the bit.

5. A lathe tool as claimed in claim 4, wherein the entire part of a bit lateral face adjoining the back of the particular main cutting edge in the direction towards the middle of the bit extends parallel to the vertical longitudinal central plane of the bit.

6. A lathe tool as claimed in claim 1 wherein the lateral faces are quite parallel. means acts on the upper face of the tool bit remote from the operative cutting edge.

7. A lathe tool as claimed in claim 1, wherein the clamping means acts on the upper face of the tool bit remote from the operative cutting edge.

8. A lathe tool as claimed in claim 1, and further comprising a recess in said tool bit at the junction of said upper faces, said clamping means engaging in said recess.

9. A lathe tool as claimed in claim 8, wherein said clamping means comprises side projections engaging said lateral faces of said tool bit.

10. A lathe tool as claimed in claim 1 and 4, wherein the lateral faces of the tool bit adjacent the cutting edges are laterally offset in opposite directions relative to the vertical longitudinal central plane of the bit.

11. A lathe tool as claimed in claim 4, and further comprising a back-pressure stop in abutment with the end of the tool bit remote from the operative cutting edge.

12. A lathe tool as claimed in claim 11, wherein the backpressure stop comprises an eccentric head of a bolt rotatably mounted transversely of said tool holder and abutting the end face of the tool bit remote from the operative cutting edge.

13. A lathe tool as claimed in claim 11, wherein the back pressure stop comprises a member freely rotatable in the tool holder about an axis substantially coincident with the cutting edge remote from the operative cutting edge and said member is provided with two abutment points abutting said tool bit.

14. A lathe tool as claimed in claim 13, wherein said back pressure stop member comprises the head of a bolt rotatably mounted in said tool holder, and a sector-shaped notch formed in said head, the side surfaces of said notch abutting the upper face and the end face of said tool bit remote from said operative cutting edge.

15. A lathe tool as claimed in claim 14, wherein said bolt head is axially bored.

16. A lathe tool as claimed in claim 4, wherein the lower face of said tool bit bounded by said shorter parallel sides of said isosceles trapezium is planar.

17. A lathe tool as claimed in claim 4, wherein the lower face of said tool bit bounded by said shorter parallel sides of said isosceles trapezium comprises two flat faces converging downwardly towards one another and joining in a straight apex line resting on said tool holder, and wherein said tool holder is provided with a co-operatingly shaped recess.

18. A bit for a lathe tool comprising a body having lateral faces which have the outline of an elongate isosceles trapezium and an elongate isosceles triangle having as its base the longer of the two parallel sides of the trapezium, the end faces of the bit forming the equal sides of the trapezium, two inclined upper faces of the bit forming the two equal sides of the triangle and a cutting edge being formed at the junction of each upper face and the adjacent end face.

19. A bit as claimed in claim 18, wherein the lateral faces are offset relatively to one another in the longitudinal direction of the bit. 

1. A lathe tool comprising, in combination:a. a tool holder; b. a tool bit on said tool holder; c. lateral faces to said tool bit, having the outline of an elongate isosceles trapezium comprising two equal sides and longer and shorter parallel sides and an elongate isosceles triangle having as its base the longer of said parallel sides; d. end faces to said bit forming the equal sides of said isosceles trapezium; e. two upper faces to said bit forming the equal sides of said isosceles triangle; f. a cutting edge to said bit at the junction of each upper face and the adjacent face; and g. means releasably clamping said tool bit on said tool holder, with one of said cutting edges projecting from said holder to form the operative cutting edge.
 2. A lathe tool as claimed in claim 1, wherein at least a portion of said lateral faces converge towards one another away from said upper faces to form side clearance angles.
 3. A lathe tool as claimed in claim 2, wherein the central portions of the laterial faces are substantially parallel to one another.
 4. A lathe tool as claimed in claim 1, wherein the laterial faces are offset relatively to one another in the longitudinal direction of the bit.
 5. A lathe tool as claimed in claim 4, wherein the entire part of a bit lateral face adjoining the back of the particular main cutting edge in the direction towards the middle of the bit extends parallel to the vertical longitudinal central plane of the bit.
 6. A lathe tool as claimed in claim 1 wherein the lateral faces are quite parallel. means acts on the upper face of the tool bit remote from the operative cutting edge.
 7. A lathe tool as claimed in claim 1, wherein the clamping means acts on the upper face of the tool bit remote from the operative cutting edge.
 8. A lathe tool as claimed in claim 1, and further comprising a recess in said tool bit at the junction of said upper faces, said clamping means engaging in said recess.
 9. A lathe tool as claimed in claim 8, wherein said clamping means comprises side projections engaging said lateral faces of said tool bit.
 10. A lathe tool as claimed in claim 1 and 4, wherein the lateral faces of the tool bit adjacent the cutting edges are laterally offset in opposite directions relative to the vertical longitudinal central plane of the bit.
 11. A lathe tool as claimed in claim 4, and further comprising a back-pressure stop in abutment with the end of the tool bit remote from the operative cutting edge.
 12. A lathe tool as claimed in claim 11, wherein the back-pressure stop comprises an eccentric head of a bolt rotatably mounted transversely of said tool holder and abutting the end face of the tool bit remote from the operative cutting edge.
 13. A lathe tool as claimed in claim 11, wherein the back pressure stop comprises a member freely rotatable in the tool holder about an axis substantially coincident with the cutting edge remote from the operative cutting edge and said member is provided with two abutment points abutting said tool bit.
 14. A lathe tool as claimed in claim 13, wherein said back pressure stop member comprises the head of a bolt rotatably mounted in said tool holder, and a sector-shaped notch formed in said head, the side surfaces of said notch abutting the upper face and the end face of said tool bit remote from said operative cutting edge.
 15. A lathe tool as claimed in claim 14, wherein said bolt head is axially bored.
 16. A lathe tool as claimed in claim 4, wherein the lower face of said tool bit bounded by said shorter parAllel sides of said isosceles trapezium is planar.
 17. A lathe tool as claimed in claim 4, wherein the lower face of said tool bit bounded by said shorter parallel sides of said isosceles trapezium comprises two flat faces converging downwardly towards one another and joining in a straight apex line resting on said tool holder, and wherein said tool holder is provided with a co-operatingly shaped recess.
 18. A bit for a lathe tool comprising a body having lateral faces which have the outline of an elongate isosceles trapezium and an elongate isosceles triangle having as its base the longer of the two parallel sides of the trapezium, the end faces of the bit forming the equal sides of the trapezium, two inclined upper faces of the bit forming the two equal sides of the triangle and a cutting edge being formed at the junction of each upper face and the adjacent end face.
 19. A bit as claimed in claim 18, wherein the lateral faces are offset relatively to one another in the longitudinal direction of the bit. 